The present invention relates to a disk array for accessing a plurality of disk units in parallel and for reading or writing data and, more particularly, to method and apparatus for taking a countermeasure of a disk array, wherein at the time of a disk failure, a spare disk unit is allocated and alternated.
The disk array accomplishes a high performance or a high reliability by combining in parallel a plurality of disk units each of which has been processed as a single physical device and by simultaneously making those disk units operative.
In order to enable data to be reconstructed at the time of a disk failure by providing a redundancy disk unit, it is important to allocate the spare disk unit on the alternative destination side of the failure disk unit. At the same time, it is required to properly allocate the spare disk unit so as not to deteriorate the performance of the disk array.
FIG. 1 shows a schematic diagram of a conventional disk array. A disk array 28 is connected as an input/output (I/O) subsystem to a host computer 10 under the domination of a controller 12. Physical positions of a plurality of disk units 30-00 to 30-35 provided in the disk array 28 are specified by ports P0 to P5 and ranks RO to R3. The ports P0 to P5 denote the device interfaces to which parallel input and output are executed from/to the controller 12. The ranks R0 to R3 denote the numbers of arrangement stages in the port direction of the plurality of disk units connected to the ports P0 to P5. The plurality of disk units 30-00 to 30-35 provided in the disk array 28 are constructed by: data disk units to store data; disk units to store parity data as redundancy data; and standby disk units as spare disk units. One parity group is constructed by every five disk units 30-00 to 30-04, 30-10 to 30-14, 30-20 to 30-24, and 30-30 to 30-34 of the ranks R0 to R3 provided for the ports P0 to P5, respectively. When considering a parity group 56 of the rank R0 as an example, in the operating format of RAID3, four disk units 30-00 to 30-03 are used for data and the remaining one disk unit 30-04 is used for parity. In the operating format of RAID5, the position of the disk unit for parity changes every sector. For example, the spare disk units 30-05, 30-15, 30-25, and 30-35 are allocated to the respective ranks R0 to R3 in which five data disk units and are parity disk unit are included.